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Cosmic collisions: astronomers seek direct observational evidence for multiple universes.

MOST PEOPLE don't wake up in the morning hoping to find a black eye. But some cosmologists would like nothing better than to spot a big, round shiner on the universe's face.

That face is the cosmic microwave background (CMB), the thermal radiation filling the entire universe. This radiation was released about 380,000 years after the Big Bang, when the fireball universe cooled down enough that electrons could combine with protons to form atoms, allowing photons to travel freely without scattering. But astronomers also think of the CMB's photons as being emitted from a surface, the outermost "edge" of the observable universe. When thought of as a surface, the CMB is our view of the set of points in space and time when the universe stopped being a superhot, ionized soup and its primordial photons scattered for the last time.

Discovered by Arno Penzias and Robert Wilson in 1964, the CMB quickly became the foremost evidence for the Big Bang. But for some time now, cosmologists have wondered whether that big bang was the only bang. Theories of modern cosmology and physics imply that our universe could be just one bubble in an endless bubble bath of universes, a landscape with the exotic name of multiverse.

The multiverse is, for now, mere speculation. But cosmologists are hoping that it won't always be. Detailed studies of the CMB's dappled surface might support this wild vision--not by letting us see other universes directly, but by recording the scars they left behind when they crashed into our cosmos.

Pocket Universes

At first glance, multiple universes seem the plaything of cosmologists who grew bored with having only one universe to toy with. But it turns out that one of the most popular frameworks of modern cosmology--inflation--usually predicts the creation of an infinity of "pocket universes" (S&T: December 2006, page 36).

Proposed by Alan Guth (now at MIT) and others at the start of the 1980s, inflation fixes several major problems with Big Bang cosmology. It explains why the universe looks uniform in all directions, why spacetime appears to be flat on large scales, and why experimentalists haven't detected huge numbers of particles with only one magnetic pole (so-called magnetic monopoles).

Inflation accomplishes this feat by introducing a cosmic hiccup (S&T: November 2005, page 32). Instead of expanding as fast as Big Bang physics predicts, the universe expanded more slowly at first, then suddenly went poof and ballooned to be at least a million billion billion times larger. The energy inherent in space itself, called vacuum energy, fueled this exponential burst, because vacuum energy exerts enormous repulsive force when concentrated. The universe expanded faster than the speed of light (Einstein's speed limit doesn't apply to spacetime itself) for at least [10.sup.-35] second, growing to be at least 1,000 times bigger than the universe we can actually observe, Guth says. After inflation ended, cosmic expansion proceeded at a normal pace.

So far, inflation has passed every observational test thrown at it with brilliant success, most notably predicting the size of warm and cool spots in the CMB. But inflation doesn't exactly tie off the universe's history with a nice tidy bow. It introduces a major conundrum: most forms of inflation are eternal. The energy that powered inflation should replenish itself as space expands, fueling an endless succession of big bangs. As Columbia University physicist Brian Greene explains in his recent book The Hidden Reality, the single point in the multiverse landscape that became our little cosmic bubble has recovered from its inflationary fever, but the multiverse as a whole might still be sick, expanding faster than any of its pockets like an ever-growing mound of Swiss cheese. As it grows, the cheese develops new holes. Inside one of these holes lies our observable universe.

"Most of the physicists who think a lot about inflation will tell you either that, 'Yeah, inflation is usually eternal and I love it,' or 'Inflation is usually eternal and I try not to think about it,'" says Anthony Aguirre (University of California, Santa Cruz). "And if we have eternal inflation, then these bubble universes are no weirder than anything else."

There's another reason the multiverse idea is popular: string theory. String theory postulates that every particle in the universe is a tiny strand of energy that vibrates in 10-dimensional spacetime. This theory remains the most detailed attempt so far to unify three sectors of physics: particle physics, Einstein's general theory of relativity (which describes gravity), and quantum mechanics (which describes particle interactions and forces at tiny scales). Despite string theory's reputation for being unverifiable with modern technologies, approaches based on its mathematical framework currently offer the best explanation to certain physical problems, including the behavior of some weird extremes of matter called perfect liquids.

Cosmologists favor string theory because it makes dark energy's value less shocking. "Dark energy" is the generic term for whatever is making cosmic expansion accelerate, but theorists' calculations show that dark energy's influence should be more than 100 magnitudes greater than observations suggest--the largest discrepancy between theory and observation in all of science.

String theory could solve this problem if multiple universes exist. The theory implies the existence of [10.sup.500] different types of empty space, with different particles, forces, and amounts of dark energy allowed in each, Guth explains. If instead of just one, every one of these [10.sup.500] possible solutions is correct--meaning each solution matches a different universe that exists in a larger multiverse--then dark energy's value isn't weird at all. We just live in one of the universes where the amount of dark energy is what we measure it to be, a value particularly friendly to our existence.

These theoretical arguments do not constitute direct evidence for multiple universes. But such evidence might be found. The infinite, higher-dimensional multiverse (the cheese in the Swiss cheese) into which these bubble universes are born would expand faster than any of its individual bubbles, but if enough universes popped into being in this landscape, some of them might form close enough to collide with our own.

When Universes Collide

This collision could leave a temperature bruise in the CMB's mottled surface shaped like a faint, round disk. Such a disk would consist of photons that are slightly warmer (or cooler) than the surrounding CMB, anomalies that are even weaker than those that show up in the iconic map from NASA's Wilkinson Microwave Anisotropy Probe (WMAP). That's saying something, because the CMB's 2.7-kelvin temperature deviates by at most 0.0002 kelvin from one point to another across the entire sky.

To understand why a cosmic collision would create a disk in the microwave background, imagine a pocket universe forming so close to ours in the multiverse that it slammed into ours like one sumo wrestler ramming into another. This violent crash would glom the universes onto each other, just like two soap bubbles stuck together. (The bubbles never bounce.) In most cases a round, soap-membrane-like wall forms between the universes. This membrane rings from the smack, and the vibration creates a wake that propagates into both universes, says Matthew Kleban (New York University), who in 2009 collaborated on the first detailed predictions of a collision's effect on the microwave background.

Our universe expands rapidly while this wave propagates through it, which dilutes the wake's energy. But the wake is still distinct enough to alter the density of the part of our universe it passes through, much as sound waves change air's density as they move from a clashing cymbal to our ears. When the universe cools enough to release photons, the wake's effect on the early universe's density shows up as a subtle, round temperature anomaly in the CMB.

If our universe actually experienced fender-benders with other bubble universes, the number of bruises our microwave background sports from these crashes will never change; it'll be the same now as in the past and the future. That's basically because from our perspective any and every collision has already happened, explains Aguirre. They happened long before the CMB formed. From the inside, the bubble's wall doesn't correspond to a point in space. Rather, it's a point in time--the Big Bang, the moment when time began. Moving farther into the bubble (as seen from the outside) corresponds to moving forward in time (as seen from the inside).

In this scenario, different eras in the universe are like a series of concentric shells. The outermost is the Big Bang. Within that shell lie the shells that mark when inflation ended, when the universe became transparent to light, and so on. A collision's effects have to pass through these time shells to reach us. Therefore from the inside, no "new" collisions ever happen. Astronomers will never look at the CMB and say, "Gee, that spot wasn't there yesterday."

By the same reasoning, none of these collisions can ever harm us. As Aguirre notes, if we're here, we've already escaped the possible consequences.

Hunting for Marks on the Sky

Cosmologists hope to detect these weak bruises on the CMB. Yet no obvious round spots appear in WMAP data of the microwave background; even the infamous Cold Spot has been dismissed as a statistical blip.

"The patterns we're talking about are covered by all sorts of random fluctuations," Kleban says. "It's like trying to watch TV with tons of static."

"There's just too much data to directly test the hypothesis," adds Matthew Johnson (Perimeter Institute for Theoretical Physics, Canada). "The WMAP experiment has something like 3 million pixels, and to test this hypothesis you need to test the correlation between each pair of 3 million pixels--which is an astronomically huge number of computations. Even modern supercomputers can't do it."

So Johnson and his colleagues have started looking for filters to weed out the gold from the sand. These filters are unique tactics of information processing that look for a predicted signal in the noise. The danger with such filters is that you can't use the method unless you know what you're looking for (in other words, the filter is designed to look for a signal that matches your theory). Johnson's team double checks itself by calculating how likely it is that the collection of candidate spots are cosmic bruises, given how the theory and data line up. If the probability is low, the candidates probably don't come from bubble collisions.

In companion papers published last year in two leading physics journals, Johnson and his colleagues described this self-check on their first generic filter attempt, called the needlet filter after the type of analysis performed. The initial run had found several anomalies in the WMAP 7-year results, but the self-check ruled out bubble collisions as the most likely explanation.

Undeterred, the group came up with a better algorithm specifically designed to look for bubble collisions. The algorithm uses so-called optimal filters, employed by various branches of physics and signal processing to detect compact objects hidden in a random background. Using this method, the team's initial results (before self-check) identified 16 candidate collisions, including all those originally found with the needlet method. The candidates range from 1.5[degrees] to 90[degrees] in angular radius. But the team hasn't run the self-check yet, so Johnson and his colleagues don't know how likely it is that the signatures are actually from cosmic smashups instead of mere statistical fluctuations.

The team is waiting on results from the European Space Agency's Planck spacecraft before it performs the double-check. Launched in 2009, Planck surveys both the temperature and polarization of the microwave background, working at an angular resolution roughly twice as fine as that of WMAP. Planck finished five full-sky surveys before its High Frequency Instrument ran out of coolant on January 14th this year. Its Low Frequency Instrument has continued working, adding data astronomers will use to improve calibration. ESA will release the first 15 months' worth of CMB data in early 2013, and the full data release will come in 2014.

It's unclear whether Planck's instruments have high enough sensitivity to detect the signatures cosmologists hope to see. Nor is it clear whether anomalies could be confidently linked to collisions or still remain vague enough to leave a lot of doubt. "I think Planck could detect something that could be strong evidence of a bubble collision," Kleban says. "Whether everyone would then believe it, I don't know. You'd have to ask everybody."

Other signals could corroborate collision candidates in the CMB. If a bulk flow of galaxies or a cosmic void lined up with these round spots, for example, that would be strong evidence in their favor. While suggestions of such phenomena have arisen, none has yet fully proven itself.

Toil and Trouble

Even assuming that eternal inflation, bubble universes, and the multiverse exist, it's possible that bubbles don't form fast enough to smack into one another before the larger multiverse's inflation carries each bubble away from every other one. So the violent-bubble-bath

picture could be true, but we might never see a mark.

"It's very much verifiable, but it's not necessarily falsifiable," Johnson cautions. And if they never detect anything? "Then we're in murky waters," he says.

"I think the primary skepticism surrounds how likely we are to see" one of these collision marks, Kleban says. "And I share that concern. I would never bet any significant amount of money that we're going to detect it, because we have to be a bit lucky."

Nevertheless, bubble collisions offer the first real possibility of finding observational evidence for something that Aguirre says his colleagues generally labeled as "a relatively innocuous pastime that will probably turn up nothing interesting."

"Years ago," he says, "when people were talking about eternal inflation and the multiverse, it was easy to dismiss that as, 'Well, that's all fun but if we can't observe it why are we bothering to think about it? This is speculation, fantasy--maybe your models predict it, but who cares?'" With the chance to observe the aftermath of a cosmic crash, bubble cosmology rises above the accusation of not being real science. Uncovering that aftermath would be a game-changing discovery.

There's also the chance that, even if cosmologists never find a CMB bruise, they'll find something else they weren't looking for. They'd be in good company: science often advances through serendipity as much as through slogging along.

"The odds are high that I'll be disappointed and we won't find these things," Kleban says. "But it's always worth pursuing these avenues, because you never really know what's there until you do."

The CMB's "surface" is somewhat like the light we see coming from a cloudy sky. When we look up at the clouds, we can only see the surface of the cloud off which light last scattered. Similarly, when WMAP observes the CMB sky, it looks back to a time in the universe when photons could scatter off free electrons.


An infinite landscape of universes is a hard thing to imagine. Although the commonly cited name for this bubble-bath landscape is "multiverse," it's just as correct to think of it as a single universe that's not the same everywhere--a universe that has regions, or "pockets," vastly different from one another. Cosmologists adopted "multiverse"rse" to make clear that this landscape is something farfar bigger and more exotic than the universe wee normally talk about. (And besides, "universe" was already taken.)

RELATED ARTICLE: Colliding Universes

From our perspective, any collision between our universe and another universe has already happened. For us, our universe's wall is not a point in space, but in time--the Big Bang, when time began. Moving from Earth into the distant universe corresponds to moving backward in time, as though through a concentric series of shells. If another universe hit ours, the wake from that collision would have to pass through these shells as it entered the universe, moving through our universe's entire history before reaching us.

RELATED ARTICLE: Rings Around the Rosies

COSMIC PUNCHES from other universes could leave a second kind of bruise in the cosmic microwave background. But this second bruise wouldn't be in the microwave background's temperature. It would show up as a unique pattern in the way the CMB's light vibrates as it propagates through space.

If a wake from a universe collision traveled through the universe, it would tweak the universe's matter density where it passed. The odd thing about these density blips is that they come in pairs. A sound wave hitting a lake's surface changes speed as it passes from air to water, because sound travels at different speeds in different media. But not all the sound hitting the water's surface passes through. Some of it reflects. The result is two acoustic waves for the price of one, with one going forward and the other backward.

Two waves will also arise in an expanding universe if the speed of sound changes everywhere in space at roughly the same time, says Matthew Kleban. That's just what happened when the universe thinned and cooled from a fireball soup to a nearvacuum. The cosmic wake is a pressure wave, like a sound wave. So when matter took over in the universe, this wave split into a reflected part and a transmitted part, leaving behind two density blips instead of one. When the photons fly soon after, they scatter off both blips.

This scattering polarizes the CMB photons, making their wavelengths wiggle in certain directions in a specific way. If the temperature bruise is about 12[degrees] across or larger, the polarization signal will appear as two rings, one inside the temperature bruise, the other outside. If observers can detect polarization signatures that match up with a temperature disk in the CMB, that would give cosmologists far more confidence that they've actually found a bruise from a colliding universe.

Big Bang                         Time        Temp (kelvin)

Inflation ends             [10.sup.-35] sec    [10.sup.19]
Atomic nuclei form                  100 sec     [10.sup.9]
CMB spectrum fixed                  1 month     [10.sup.7]
Radiation balances matter         10,000yrs         20,000
CMB last scattering            380, 000 yrs          3,000

Present 13.7 billion years after the Big Bang

Assistant editor Camille M. Carlisle is a fond denizen of our cozy little cosmos and, frankly, plans to keep her sights within its confines until evidence demands otherwise.
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Title Annotation:Cutting-Edge Cosmology
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Dec 1, 2012
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